If you need to remount a in-use filesystem like / or /usr without rebooting the operating system, use the mount command with the -o remount option.

To remount the root partition using settings found in /etc/fstab.conf run:

mount -o remount /

This is handy if a filesystem has been mounted read only during a recovery process. For example to remount the root partition read/write do:

mount -o remount,rw /

If there is no /etc/fstab entry for the root partition (or any partition you are using) you may need to specify the root partition (/dev/sda1 in this example):

mount -o remount,rw /dev/sda1 /